1. Field of the Invention
The present invention relates to an electric potential measuring apparatus, an image forming apparatus using the electric potential measuring apparatus, an electric potential measuring method, an electrostatic capacitance measuring apparatus, and an electrostatic capacitance measuring method.
2. Description of the Related Background Art
Conventionally, there exists an image forming apparatus that includes a photosensitive body and forms an image in an electrophotographic manner. In such an image forming apparatus, to form a high-quality image, the apparatus needs to be controlled while the electric potential of the photosensitive body is measured. To achieve the above-stated purpose, there has been proposed an example of the electric potential measuring apparatus in which a detecting electrode is located close to an electrically-charged photosensitive body (an object being measured, or a measurement object), the capacitance between the photosensitive body and the detecting electrode is mechanically changed as below-described, and a small amount of charges electrostatically induced in the detecting electrode is measured.
FIG. 12 shows a conceptual structure of an electric potential measuring apparatus. As illustrated in FIG. 12, a measurement object 501, a detecting electrode 502, and a charge detector 503 are arranged. In FIG. 12, reference character VD designates a surface electric potential of the measurement object 501. Reference character C1 designates a capacitance to be changed by a capacitance changing unit (a capacitance modulator) for changing the electrostatic capacitance between the photosensitive body 501 and the detecting electrode 502. Reference character VOUT designates a signal output from the charge detector 503.
As a method of mechanically changing the capacitance C1, there have been proposed a method in which the number of electric lines of force reaching the detecting electrode 502 from the measurement object 501 is periodically changed, and a method in which the detecting electrode 502 are periodically displaced.
U.S. Pat. No. 4,720,682 discloses a structure in which a fork-shaped shutter is interposed between a measurement object (a photosensitive body) and a detecting electrode, and the shutter is periodically moved in directions parallel with a surface of the measurement object such that electric field lines of force from the measurement object are periodically prevented from reaching the detecting electrode. The effective area of the detecting electrode viewed from the measurement object is thus changed, and the electrostatic capacitance between the measurement object and the detecting electrode is hence varied so that the electric potential of the measurement object can be detected.
Further, there has been proposed a structure in which a metal shielding member with an opening is arranged facing a measurement object, and a detecting electrode is disposed on an end portion of a fork-shaped vibrating member. U.S. Pat. No. 3,852,667 discloses a structure in which the detecting electrode is moved in a parallel manner right under the opening of the vibrating member, and the number of electric lines of force reaching the detecting electrode is accordingly changed so that the electrostatic capacitance is modulated.
U.S. Pat. No. 4,763,078 discloses a structure in which a detecting electrode is arranged on an end portion of a vibrator in the form of a cantilever that is vibrated to change the distance between a measurement object and the detecting electrode, and the electrostatic capacitance is thus modulated.
In the above-described electric potential measuring apparatuses, or electrostatic capacitance measuring apparatuses, the condition of the vibration for modulating the electrostatic capacitance must be detected to achieve a stabilized vibration of the fork-shaped shutter, or the vibrator. However, a sensor for detecting the vibration needs to be additionally arranged to detect the condition of the vibration, leading to an increase in the number of constituent components.